<script>
    function captcha(){
        var captcha = '123456789abcdefghijklmnopqrstuvwxyz';
        var one = Math.floor(Math.random()*(captcha.length-1-0+1)+0);
        var two = Math.floor(Math.random()*(captcha.length-1-0+1)+0);
        var three = Math.floor(Math.random()*(captcha.length-1-0+1)+0);
        var four = Math.floor(Math.random()*(captcha.length-1-0+1)+0);
        console.log(captcha[one],captcha[two],captcha[three],captcha[four]);
    }
    captcha();
</script>